(a) Field of the Invention
The present invention relates to an apparatus for treating (causing a chemical reaction or merely heating) carbon containing compounds such as hydrocarbons or their derivatives or carbon monoxide or the like at temperatures higher than about 500.degree. C.
(b) Description of the Prior Art
As the materials for constructing the above mentioned apparatus for treating carbon containing compounds, steels and Ni alloys have usually been largely used. Therefore, carbon deposition frequently occurs on the portions exposed to the high temperature fluid of carbon containing compounds in heater tubes, piping, fractionators, heat exchangers and the like during operation. Accordingly, various operational ill effects such as rise in .DELTA.P, reduction in heating efficiency and the like are often caused, thereby making it necessary to perform so-called decoking very frequently. It may be said that this decoking operation impedes the steady running of the apparatus and further acts not only to aggravate the economy of the process but also to exert various disadvantages upon the construction materials of the apparatus.
Cr is normally added to the construction materials of these apparatuses, namely steels or Ni alloys, from the viewpoint of corrosion resistance. The Cr contents thereof are less than 28 wt. %, where the Cr contents of the usual heat resisting steels and alloys are about 25 wt. %. Because of this, a protective oxide film such as Cr.sub.2 O.sub.3 film is formed on the surface of these materials in the initial stage. However, since the operating environment comprises a carburizing/oxidizing atmosphere with thermal cycles in the actual apparatus, the Cr contained just beneath the surface is consumed sooner or later thereby causing deterioration of the material surface for this level of Cr content. Consequently, oxides of Fe and Ni such Fe.sub.2 O.sub.3, NiO (or spinel oxides such as NiFe.sub.2 O.sub.4, FeCr.sub.2 O.sub.4, NiCr.sub.2 O.sub.4 and the like) and so forth appear on the outer surface. These oxides of Fe and Ni are easily reduced by carbon containing compounds into metallic Fe and Ni, thereby causing carbon deposition.
According to the report of Lobo and others (Preprint for the 5th International Congress on Catalysis, Amsterdam (1972)), it is concluded that carbon deposition is caused by the transition metal elements, such as Fe, Co, Ni and the like, and the said carbon deposition is continued by their atoms and metal particles ceaselessly appearing, as if floating, on the upper surface of the carbon deposit layer.
Since it is actually proved by the present inventors' investigation that according to their analyses of the coke deposited on the inner surface of the member of the apparatus, transition metal elements such as Fe, Ni and the like can be detected, it is conjectured that carbon deposition is attributable to the supply of transition metal elements such as Fe, Ni and the like, brought about by reduction of the oxide containing Fe, Ni and the like as its constituent elements on the inner surface of the member or by diffusion of said elements through the surface oxide layer from the interior of the member wall.
In order to prevent carbon deposition in these apparatuses, various investigations have been carried out. For instance, it is reported in "Ind. Eng. Chem. Proc.-Design and Development. 8 [1] (1969) 25 by B. L. Crynes, L. F. Albright" that carbon deposition in ethylene producing apparatus can be somewhat suppressed by adding a very small amount of H.sub.2 S to the feed, and some processes are employing this. However, the fact is that since the inside of the cracking tube member used in an ethylene producing apparatus or the like is under an extreme oxidizing atmosphere from the very beginning, it is difficult to sulfurize the metal surface and so sufficient effects are not achieved. In addition, some methods of preventing carbon deposition by utilizing an Al and/or Al oxide layer or film have been proposed whereby said layer or film covers the transition metals which promote carbon deposition such as Fe and Ni contained in the material in order to prevent those metal elements from contacting directly with carbon containing substances. Among them are the idea of hot-dipping the surface of the construction material with Al melt (U.S. Pat. No. 3,827,967) or calorizing (diffusing and penetrating Al) the surface of the construction material (L. F. Albright et al : "Thermal Hydrocarbon Chemistry", ACS Adv. Chem. Ser. 183; M. Papapietro et al: "Symposium on Coke Formation on Catalysts in Pyrolysis Units", ACS New York Meeting, Aug. 23-28 (1981) 723), and the apparatus with resistance to carbon deposition which comprises forming an Al oxide film on the Al increased surface of the construction material which has previously been alloyed with Al to such an extent that the material preserves its ductility and further has been enhanced in Al content by aluminizing its surface (Japanese Laid Open Patent Application 25386/1982).
However, these proposals still include the undermentioned problems. Namely, although the outermost surface matter possesses a sufficient capability to prevent carbon deposition in the beginning, the effect is liable to diminish sooner or later, because the surface metallurgically deteriorates on account of the secondary diffusion of Al in long-term use at elevated temperatures under a carburizing/oxidizing atmosphere which is subject to thermal cycles. Also, alloy materials containing much Al are inadequate for use as tube materials, because they are too brittle at ambient temperatures.